Provide a way for plugins to override classes.

jquery.flot.js

@@ -541,9 +541,16 @@ Licensed under the MIT license.
         }
 
         function initPlugins() {
+
+            // References to key classes, allowing plugins to modify them
+
+            var classes = {
+                Canvas: Canvas
+            };
+
             for (var i = 0; i < plugins.length; ++i) {
                 var p = plugins[i];
-                p.init(plot);
+                p.init(plot, classes);
                 if (p.options)
                     $.extend(true, options, p.options);
             }